Short answer: size, API and ease of resource management.
My comment was more about the state then than the state now. At that time few, if any languages were designed from scratch to be extended and embedded. Tcl and Guile, to my knowledge were among the first. Tcl interpreter is a library. IMO Guile has grown beyond that charter and is more of a stand-alone Scheme now, but one that can be easily embedded / extended, but so is Chicken I guess.
My comment was more about the state then than the state now. At that time few, if any languages were designed from scratch to be extended and embedded. Tcl and Guile, to my knowledge were among the first. Tcl interpreter is a library. IMO Guile has grown beyond that charter and is more of a stand-alone Scheme now, but one that can be easily embedded / extended, but so is Chicken I guess.